As a conventional method of predicting reproduction colors of an image generated by subtractive color mixing (e.g., a print process), reproduction color prediction using a lookup table (to be abbreviated as an LUT hereinafter) disclosed in Japanese Patent Laid-Open No. 2001-053976, and a reproduction color prediction method using the Kubelka-Munk theory (to be abbreviated as a KM theory hereinafter) disclosed in Japanese Patent Laid-Open No. 09-120185 are known.
In reproduction color prediction using an LUT, a large number of patches formed by changing step by step the dot quantity of ink used in a print process are output, and the obtained colorimetric data are geometrically laid out on a color space such as CIELAB, as shown in FIG. 10 (each vertex of cubes shown in FIG. 10 stores a colorimetric value and the dot quantity of each ink in correspondence with each other). After that, an ink dot quantity corresponding to a desired tristimulus value (a point indicated by an open circle in FIG. 10) is interpolated on the basis of the geometrical layout with neighboring existing points (points indicated by full circles in FIG. 10), thus calculating a desired dot quantity.
The KM theory examines I, ΔI, J, and ΔJ with respect to infinitesimal thickness dx in ink, as shown in FIG. 11, and calculates reflectance (J/I) by solving:dI=−(S+K)Idx+SJdx  (1)dJ=(S+K)Jdx−SIdx  (2)                S: scattering coefficient of ink        K: absorption coefficient of ink        
In reproduction color prediction using an LUT, the number N of patches that must be output to generate an LUT is given by:
                    N        =                              (                                          100                P                            +              1                        )                    I                                    (        3        )                            N: number of patches to be output        P: interval (%) upon changing dot quantity        I: number of inks used        
Therefore, when interval P upon changing the dot quantity is decreased or when the number I of inks used upon executing a print process using multi-color inks is increased to improve the prediction precision, the number N of patches to be output increases exponentially, resulting in huge cost of output and colorimetry.
The KM theory predicts reproduction colors when a coloring material such as ink is applied to have a uniform thickness. Therefore, when a print process is made using an area-modulation printer shown in FIG. 12, a mechanical dot gain (a phenomenon that the effective area ratio becomes larger than the theoretical area ratio due to physical spread of ink) and an optical dot gain (a phenomenon that an actual dot looks larger than its original area due to scattering of light in ink or paper) which occurs at the boundaries between portions with and without ink cannot be precisely predicted.
For example, the reproduction color-prediction method of Japanese Patent Laid-Open No. 09-120185 expands the KM theory to apply it to an actual printer, and predicts reproduction colors by independently modeling a portion where a plurality of inks mix, and a portion where a plurality of inks overlap each other. However, since this method does not consider the influence of an optical dot gain, it cannot implement precise reproduction color prediction.
A general color print is printed by a process print method, which uses a total of four color inks (C, M, Y, and. K), i.e., three color inks cyan, magenta, and yellow that are generated from a color document via three-primary color separation, and black. When an identical image is to be printed in large quantity like those of magazines, posters, and the like, a print process is made by adding several different special color inks suited to that original image, thus realizing delicate color appearance and the color gamut that cannot be reproduced by the process print. For example, upon developing an ink-jet or laser printer, C, M, Y, and K inks are normally used. However, C, M, Y, and K inks of various characteristics are available, and many companies have addressed development of inks with higher quality. A technique that adds another ink in addition to the C, M, Y, and K inks, and prints using five or more inks has been studied. In order to improve such ink development efficiency, it is demanded to automatically optimize inks.
For this purpose, recently, a method of automatically and precisely making color separation into respective plates upon using special color inks has been developed. For example, Japanese Patent Laid-Open No. 2001-053976 discloses a special color color-separation method for separating an original image into Y, M, and C plates and a special color plate. On the other hand, as a technique for improving the color reproduction precision, a spectral color reproduction technique that matches spectral distributions themselves in addition to the tristimulus values of colors has been disclosed in Japanese Patent Laid-Open No. 05-296836. In this way, in order to precisely reproduce a target color, there are two different approaches, i.e., a method of using a special color ink (special color color-separation method) and a method of making spectral distribution characteristics as closer as possible although conventional inks are used (spectral color reproduction).
The conventional special color color-separation method makes color separation for given C, M, Y, and K inks and special color ink. For example, Japanese Patent Laid-Open No. 2001-053976 requires colorimetric data of the special color ink for color separation, and is premised on the use of the special color ink manually selected in advance. However, as for a selection method of inks themselves, i.e., a method that specifies combinations of inks which allow optimal color reproduction, no clear method is established yet. For this reason, a skillful engineer selects special color ink by trial and error in practice.
On the other hand, in spectral color reproduction, a spectral distribution is made closer to that of a target color using given inks so as to realize color reproduction closest to the target color. However, it is impossible for spectral approximation to reproduce spectral distribution characteristics of a target image or color using given inks alone. Furthermore, no technique that specifies inks of spectral distribution characteristics that can reproduce those of a target color/image is available.
As described above, a printer as an image output apparatus normally outputs an image using C, M, Y, and K inks (or toners) if it is a four-color printer. A six-color printer outputs an image using two light inks or special color inks in addition to the above four colors. Note that the color gamut of the printer is determined by the colors of color agents such as inks, toners, and the like.
In general, as a method of measuring the color gamut of an image generated by subtractive color mixing (e.g., a print process), for example, a method of approximating the color gamut using a polynomial of higher degree, as disclosed in Japanese Patent Publication No. 63-32313, a method of approximating the color gamut using a neural network, as disclosed in Japanese Patent Laid-Open No. 2-241271, and the like can be used. Also, a method of generating a device model using a method of generating a plurality of patches and predicting the color gamut using the weighted mean of colorimetry results of these patches is available, as disclosed in Japanese Patent Laid-Open No. 10-262157.
As described above, Japanese Patent Laid-Open No. 09-120185 describes the color reproduction prediction method using the KM theory.
However, the aforementioned polynomial of higher degree, neural network, and device model based on the weighted mean normally requires a huge number of patches to attain gamut prediction with higher precision. The KM theory cannot precisely predict a mechanical or optical dot gain if a print process is made using an area-modulation printer, as shown in FIGS. 11 and 12.
As described above, upon printing an identical image in large quantity like those on magazines, posters, and the like, a print process is made by adding several different special color inks suited to that original image so as to reproduce delicate color appearance or the color gamut that cannot be reproduced by process print. Recently, a method of automatically and precisely making color separation into respective plates upon using special color inks has been developed. For example, Japanese Patent Laid-Open No. 2001-053976 discloses a special color color-separation method for separating an original image into Y, M, and C plates and a special color plate.
As described above, as for a selection method of inks themselves, i.e., a method that specifies combinations of inks which allow optimal color reproduction, no clear method is established yet. For this reason, a skillful engineer selects special color ink by trial and error in practice.
Upon reproducing a color which cannot be reproduced by conventional inks, it is difficult to estimate the characteristics of inks to be used.